It is generally well known that magnetic survey tools are disturbed in varying ways by anomalous magnetic fields associated with fixed or induced fields in elements of the drill string. It is further well known that the predominant error component lies along the axis of the drill string. This latter fact is the basis for several patented or patent-applied-for procedures to eliminate the along-axis field errors in three-magnetometer survey tools. Among these are U.S. Pat. No. 4,163,324 to Russell et al; U.S. Pat. No. 4,433,491 to Ott et al; U.S. Pat. No. 4,510,696 to Roesler; U.S. Pat. No. 4,709,486 to Walters and U.S. Pat. No. 4,818,336 to Russell; and applications for U. K. Patents 2,138,141A to Russell et al; and U.S. Pat. No. 2,185,580 to Russell; as well as European application 0 193 230 and U.S. Pat. No. 4,682,421 to Van Dongen.
All of these methods, in effect, ignore the output of the along-axis magnetometer, except perhaps for selecting a sign for a square root computation. They provide an azimuth output by computation of a synthetic solution, either:
1) by using only the two cross-axis magnetometers and known characteristics of the earth's field, or
2) by using the cross-axis components and an along-axis component computed from the cross-axis components and known characteristics of the earth's field.
Most of these methods require, as the known characteristics of the earth field, one or more of the following:
1) Field Magnitude
2) Dip Angle
3) Horizontal Component
4) Vertical Component
The Walters method requires, as known characteristics of the earth field, only that:
1) The Field Magnitude is constant in the survey area. PA1 2) The Dip Angle is constant in the survey area. PA1 i) two cross-borehole components, PA1 ii) two cross-borehole components and an along-borehole component,
The fact that these quantities are constant is all that is required. The value of the constant is not needed but is derived within the correction algorithm.
It may be shown that in all of the individual methods of the above references, the final error in the computed azimuthal orientation of the borehole axis is completely independent of the along-borehole magnetic measurement and therefore the along-borehole component of the drill string interference. This is true because that measurement is simply not used in any manner that affects the final computed result. However, it may also be shown that all of the cited methods introduce other errors that are functions of the sensor errors for those sensor outputs used, errors in the reference information related to the earth's magnetic field used in the solutions, and the orientation of the borehole axis in azimuth and inclination. These factors lead to a result that no single method of compensation for drill string interference will provide the smallest error for all orientations of the borehole axis. Further, the complexity of the error relations for the various individual methods leads to a difficult problem for survey operators to understand the error regions and magnitudes. Of particular concern is that as the borehole may progress, the changes in borehole orientation cause different errors at each survey station. Often, no single method can provide minimum error for all stations along the path.